Semi-finished lenses using thermoplastic resins save cost and time for the molding and thus are useful. In particular, the polycarbonate semi-finished lenses having impact resistance can resist the impact by a foreign object to protect the eyeballs when the eyeglass frame is worn and thus are useful.
As the thermoplastic resins other than the polycarbonate, polyamide, modified polycarbonate-polyester, thermoplastic polyurethane, and the like also have the impact resistance and thus are suitable. Furthermore, in the case that a composite lens is prepared, when the same resins are used, a special adhesive is not required for the adhesion of both layers. Furthermore, even in the case that the same resins are not used, when resins each having a similar refractive index are used, the adhesion of both layers can be improved and the lens distortion can be prevented as much as possible.
There are the following four methods for manufacturing such tinted semi-finished lenses for sunglasses for light shielding. However, each method has problems.
Method a): The back face of a lens is cut and polished according to the visual acuity of a user, and then the lens is wholly immersed in a stain solution to stain the front and back faces of the lens. In the method, the lens is required to be polished according to a user, and the lens must be stained after the polish. Thus, it needs high cost as well as takes too long time for the lens preparation.
Method b): From the beginning, a whole lens including the front and back faces is stained, and the back face is cut according to the visual acuity of a user. In the method, because the front and back faces are similarly stained, it is unknown how diluted color the lens has by polishing until the polishing is finished. In addition, because the PC material is less stained, when the staining with high density is required, the immersion at high temperature is required to cause the problem of a variable front curve.
Method c): A colorant is mixed with pellets at the time of lens molding to wholly stain a lens with uniform density. In the method, because the back face of the lens is polished according to each user, when the periphery part of the lens has a different thickness from the central part, the absorptivity cannot be constant, for example, the transmission factors are different. In particular, the lens having high light-shielding (low absorptivity) has the problem because the density difference is remarkable.
Method d): A bending process is performed on a tinted sheet on the front curve of a lens to be formed, and the tinted layer is integrally formed on the front face of the lens by an insert molding. The method has problems. For example, because the tinted sheet is required to be separately prepared, it is difficult to respond to color variations. Moreover, because the tinted sheet is required to be cut into a lens shape and to be processed with bending, the lens becomes expensive.